1. Field of the Invention
The present invention relates to a piston structure, and more particularly to piston structure which is free of seal ring.
2. Description of the Prior Art
When moving an object upward or downward, a jack or a hydraulic cylinder will be used to save strength. When the hydraulic force of the jack is used, due to the movement of a piston inside the cylinder, resistance from the fluid inside the cylinder provides the necessary force to raise the object such as a tool head to serve as a counter balance cylinder. With reference to FIG. 7, a conventional piston (41) inside a cylinder (40) is shown. The piston (41) is movably received inside the cylinder (40) and due to the existence of the piston (41), an interior of the cylinder (40) is divided into two compartments (400,410). Furthermore, in order to prevent leakage of fluid inside the cylinder (40), a seal ring (43) is provided at both a top portion and a bottom portion of an outer periphery of the piston (41). A friction element (44) is normally mounted between the two seal rings (43) and on the outer periphery of the piston (41) to maintain stable axial movement of the piston (41) inside the cylinder (40). Thus when the piston (41) is moved, movement of the fluid inside the cylinder (40) from one compartment (400 or 410) to the other compartment (410 or 400) provides the necessary force to accomplish the designated tasks.
However, due to the friction between the friction element (44) and the inner side wall of the cylinder (40) and the friction between the seal rings (43) and the inner side wall of the cylinder (40), part of the force driving the piston (41) is used to overcome the above mentioned friction. Furthermore, if the friction element (44) or the seal rings (44) are worn, leakage of the fluid may cause replacement of the entire cylinder.
To overcome the shortcomings, the present invention tends to provide an improved piston structure to mitigate the aforementioned problems.